diff --git a/fck/cktype/__init__.py b/fck/cktype/__init__.py
index 8622dfe..0dd0dec 100644
--- a/fck/cktype/__init__.py
+++ b/fck/cktype/__init__.py
@@ -18,12 +18,14 @@ def resolve(fname: str, esum: Optional[str] = None, cstype: str = ""):
     If neither is applicable it defaults to CRC32.
     """
     cstype = cstype.upper()
-    if cstype in CKTYPES:
-        if esum is None and (match := CKTYPES[cstype].REGEX.search(fname)):
-            esum = match.group(0)
-        return CKTYPES[cstype](), esum
+    if cstype != "":
+        if cstype in CKTYPES:
+            if esum is None and (match := CKTYPES[cstype].REGEX.search(fname)):
+                esum = match.group(0)
+            return CKTYPES[cstype](), esum
+        raise ValueError(f"Unknown checksum type: {cstype}")
 
-    for cstname, csclass in CKTYPES.items():
+    for csclass in CKTYPES.values():
         if match := csclass.REGEX.search(fname):
             if esum is None:
                 esum = match.group(0)